One centilitre is exactly 10 millilitres. The centilitre is little used in English but standard on drinks labels across continental Europe, which is why a wine bottle says 75 cl rather than 750 ml.
About the units
The centilitre is a hundredth of a litre, from the Latin centum. Like the centimetre it survives despite SI's preference for steps of a thousand, and for the same reason: it matches a practical scale. Drink servings fall naturally in the range of a few centilitres, and printing 75 cl on a bottle is tidier than 750 ml. Sweden, France, Italy and Spain use it routinely on labels and in bar measures, while the United Kingdom and the United States use millilitres and fluid ounces respectively for the same quantities.
The exact factor
The factor is exactly 10, a single decimal place. 4 cl is 40 ml; 250 ml is 25 cl. The family relationships are worth having together: 1 litre is 100 cl, 10 dl and 1,000 ml. A standard spirit measure in Sweden is 4 cl, in much of Europe 2 or 5 cl, and in the United Kingdom 25 or 35 ml — so a cocktail recipe written in one convention needs converting before it can be made with the other's equipment.
Where you meet this conversion
Bottle and can labelling across continental Europe, bar measures, and cocktail recipes. Swedish and French drink recipes are written in centilitres throughout. Perfume and cosmetics use millilitres, so the two appear side by side in the same shop. In cooking the centilitre is used occasionally in Swedish recipes, sitting between the millilitre and the deciliter, though the deciliter dominates.